A multitude of wired and wireless access technologies are now available or in development for connecting "things." Prior to examining these access technologies, it is essential to discuss the criteria for evaluating them across different use cases and system solutions.
Wireless communication is ubiquitous in smart object connectivity, mostly due to its facilitation of deployment and the mobility it affords, enabling smart devices to relocate without losing connectivity. The subsequent parts consider this as they examine various criteria. Furthermore, considerations regarding wired connectivity are addressed when relevant.
Range
When discussing wired and wireless access technologies, it is important to consider the distance between devices and whether indoor or outdoor deployments should be differentiated. Wireless technologies can be categorized into short range, medium range, and long range.
- Short-range technologies, such as IEEE 802.15.1 Bluetooth and IEEE 802.15.7 Visible Light Communications (VLC), support tens of meters of maximum distance between devices.
- Medium-range technologies, such as IEEE 802.11 Wi-Fi and IEEE 802.15.4, have a maximum distance of less than 1 mile.
- Long-range technologies, such as cellular (2G, 3G, 4G) and outdoor IEEE 802.11 Wi-Fi and Low-Power Wide-Area (LPWA) technologies, are ideal for battery-powered IoT sensors.
Frequency Bands
Radio spectrum is regulated by organizations like the ITU and FCC, defining regulations and transmission requirements for various frequency bands. It is a critical resource for various communications uses, such as radio, television, and military. Licensed spectrum is used for IoT access technologies, with licensed bands being used by services providers, public services, broadcasters, and utilities.
Users must subscribe to services when connecting devices, adding complexity to deployments. IoT platforms like Cisco Jasper Control Center automate provisioning, deployment, and management of large numbers of devices in licensed spectrum.
The ITU defines unlicensed spectrum for industrial, scientific, and medical (ISM) frequencies in radio bands, used in short-range devices for IoT access without guarantees or protections. The bands are as follows:
- 2.4 GHz band as used by IEEE 802.11b/g/n Wi-Fi
- IEEE 802.15.1 Bluetooth
- IEEE 802.15.4 WPAN
Countries may also specify other unlicensed bands. For example, China has provisioned the 779-787 MHz spectrum as documented in the LoRaWAN 1.0 specifications and IEEE 802.15.4g standard.
Power Consumption
IoT devices are divided into powered and battery-powered nodes. Powered nodes have direct connections to a power source, but their deployment is limited by power availability. Battery-powered nodes offer more flexibility and are classified by battery life requirements.
IoT wireless access technologies must address low power consumption and connectivity for battery-powered nodes, leading to the evolution of Low-Power Wide-Area (LPWA). Power optimization is essential for wired IoT access technologies, such as smart meters, which require a minimum of 5-10 watts of power for communication.
Topology
IoT devices can be connected using three main topology schemes: star, mesh, and peer-to-peer. Star topologies are common for long-range and short-range technologies, using a single central base station or controller. Medium-range technologies often use star, peer-to-peer, or mesh topologies. Peer-to-peer topologies allow devices to communicate as long as they are in range.
Indoor Wi-Fi deployments typically use a star topology around access points, while outdoor Wi-Fi may use a mesh topology for the backbone. Mesh topology helps cope with low transmit power, search for greater distance, and coverage by having intermediate nodes relay traffic for other nodes. However, mesh topology requires optimized implementation for battery-powered nodes, as they may be placed in a "sleep mode" to preserve battery life.
Constrained Devices
The Internet Engineering Task Force (IETF) defines constrained nodes in RFC 7228, distinguishing them from unconstrained nodes like servers and mobile devices. These nodes have limited resources, impacting their networking capabilities and not implementing an IP stack.
Constrained-Node Networks
IoT access technologies like Wi-Fi and cellular are suitable for connecting constrained nodes, such as IEEE 802.15.4 and 802.15.4g RF, IEEE 1901.2a PLC, LPWA, and IEEE 802.11ah. These networks are often referred to as low-power and lossy networks (LLNs), as they must cope with power and battery requirements. Layer 1 and Layer 2 protocols must be evaluated for use-case applicability.
Data Rate and Throughput
IoT access technologies offer data rates ranging from 100 bps to tens of megabits per second, but actual throughput is often less than the data rate. Understanding the bandwidth requirements, capacity planning rules, and expected real throughput is crucial for network design and successful production deployment.
Technologies not specifically designed for IoT, such as cellular and Wi-Fi, match well with high bandwidth applications like video analytics.
Short-range technologies, like Bluetooth sensors, provide medium to high data rates with enough throughput to connect a few endpoints.
IoT access technologies for constrained nodes are optimized for low power consumption but are limited in terms of data rate and throughput.
Latency and Determinism
Latency expectations in IoT applications should be considered when choosing an access technology, especially in wireless networks. Constrained networks may have latency ranging from milliseconds to seconds, requiring applications and protocol stacks to cope. UDP is recommended for IP endpoints and routing optimization in mesh topologies.
Overhead and Payload
When considering constrained access network technologies, it's crucial to consider MAC payload size characteristics and IPv6 fragmentation requirements. The minimum IPv6 MTU size is 1280 bytes, so link layer access protocols with smaller MTUs must consider this. Layer 1 or Layer 2 fragmentation capabilities and/or IP optimization are essential for technologies that can transport IP. Most LPWA technologies offer small payload sizes to cope with low data rate and time over the air requirements of IoT nodes and sensors. These payloads are better suited for Class 0 and 1 nodes, as defined in RFC 7228.
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